The present invention relates to a system for controlling the transmission ratio of an infinitely variable belt-drive transmission for a vehicle. U.S. Pat. No. 4,152,947 discloses a control system for an infinitely variable transmission.
The infinitely variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool to control the oil pressure by shifting the spool.
The primary pulley has a Pitot pressure generating device for producing Pitot pressure dependent on engine speed. The Pitot pressure is applied to one axial end of the spool of each valve to urge the spool. On the other hand, the actual transmission ratio is detected by the axial position of the movable conical disc of the primary pulley which represents the running diameter of the belt on the primary pulley. The position of the movable conical disc is transmitted to the other axial end of the spool of the pressure regulator valve by a rod and a link.
The spool of the transmission ratio control valve is shifted in dependency on the opening degree of a throttle valve of an engine and on the engine speed (Pitot pressure) to control the amount of oil to be supplied to the servo device of the primary pulley so as to control the transmission ratio. The spool of the oil pressure regulator valve is shifted in dependency on the engine speed and on the transmission ratio to regulate the line pressure of the oil. Thus, the transmission ratio can be smoothly and infinitely varied.
In such a system, when starting a vehicle provided with the transmission, the transmission ratio is decided to be a predetermined maximum value and the value can not be changed in accordance with the starting conditions, for example under a heavy load or light load condition such as starting on a downhill. Accordingly, the transmission is inevitably upshifted from the maximum ratio when starting the vehicle and downshifted to the maximum ratio at deceleration regardless of the conditions of load. This results in a decrease of driveability and an increase of fuel consumption.